Glue guns for manual operation typically include a tube shaped heating chamber and frequently a seal for the entering glue stick at its rear end and a nozzle on its front end for discharging the melted material. A housing is generally included which has a hand grip for the convenience of the user. As the solidified glue stick is advanced into the heating chamber, the portion of the glue stick which is in the heating chamber is heated to beyond its melting point, and the remainder of the still solid glue stick acts as a piston to push the melted glue out the nozzle and onto the work piece. In early forms of glue guns, the glue stick was advanced directly by the user, such as by pressing directly on the back end of the glue stick with the user's thumb.
Pressing directly on the glue stick has problems, however, since the glue stick has a tendency to become hot during operation of the device. Furthermore, on occasion, due to either failure by the user to allow sufficient time for the heating chamber to warm up, or the user applying the glue at a rate too high to allow adequate melting of the glue stick in the heating chamber, the glue may be inadequately melted in the heating chamber. If the glue is inadequately melted, and the user continues to attempt to force glue through the heating chamber when it is still in a solid or semi-solid state, excessive force may be imparted to the parts of the mechanism and to the heating chamber, causing damage.
Various types of advancing mechanisms which advance the glue stick into the heating chamber without the need for the user to directly press on the glue stick, and which prevent excessive force, have been proposed. Prior art glue stick advancing mechanisms include those having a slidable carriage member holding the glue stick with an angularly rotatable gripping member pivoted about a hinge pin attached to the carriage member, such as described in U.S. Pat. No. 4,523,705, issued June 18, 1985. As described therein, the gripper member is connected to a tension spring which is, in turn, connected via a cable and pulley arrangement to a sliding trigger. Squeezing the trigger pulls the cable around the pulley which, in turn, pulls on the tension spring which then pulls on the gripper member to cause angular rotation of the gripper member about the hinge pin. This forces a corner of the gripper member against the glue stick, thereby clamping the glue stick in the carriage member. Continued application of force to the gripper member causes the carriage to slide longitudinally forward with the glue stick to advance the glue stick into the heating chamber. If excessive forces are applied, the spring expands, limiting the force which can be applied to the gripper member.
While these prior art glue guns are satisfactory in many applications, the present design of the glue advancing mechanism has difficulties. Attachment of the gripper to the slidable carriage with a hinge pin increases the cost of the mechanism if a metallic hinge pin is used for strength. If the hinge pin is molded of plastic to decrease cost, the strength of the hinge pin may not be adequate. Use of a cable and pulley arrangement for attachment to the tension spring, although satisfactory from the standpoint of preventing excessive force, is mechanically complicated and hence expensive.